[mkgmap-dev] Complex ocean polygon hiding the islands

Sat Feb 28 12:28:07 GMT 2015

Gerg,

Once again, thank you very much for you prompt, super detailed, complete
and instructive answer.

Best regards,

Alexandre

2015-02-28 4:45 GMT-03:00 Gerd Petermann <gpetermann_muenchen at hotmail.com>:

> Hi Alexandre,
>
> yes, simplification happens in various places. The reasons are limits in
> the Garmin
> format as well as optimization of the img file size.
>
> 1) The Garmin img format uses a raster with ~2.3 m distance at resolution
> 24.
> I think of it as a bed of nails, you can draw straigt lines between any of
> the nails.
> Details below this resolution are removed. We use two different algos for
> lines and shapes
> here, so expect slightly different results for outlines.
> 2) The Garmin format doesn't allow a single polygon with more than 255
> points, so mkgmap
> has to split a complex polygon into smaller pieces.
> 3) For lower resolutions, the raster is less precise, means, for 23 points
> are ~4.6m, 22 -> ~9.6m and
> so on.
> 4) For lower resolutions, a variant of the Douglas-Peucker-Algo is used to
> reduce the img size.
> You can change the effect of this by using the "Optimization options":
> http://www.mkgmap.org.uk/doc/options
>
> See also
>
> http://en.wikipedia.org/wiki/Ramer%E2%80%93Douglas%E2%80%93Peucker_algorithm
>
> Gerd
>
> ------------------------------
> Date: Sat, 28 Feb 2015 01:33:18 -0300
> From: alexandre.loss at gmail.com
> To: mkgmap-dev at lists.mkgmap.org.uk
> Subject: [mkgmap-dev] Complex ocean polygon hiding the islands
>
>
> Hi guys,
>
> Please see if any of you can help me ...
>
> I have an ocean polygon with multiple edges and in the middle of these
> there are several islands that are in fact holes in the polygon.
> Here's an example:
>
> [image: Imagem inline 1]
>
> Showing only the contour lines, the polygon is drawn as follows:
>
>
> [image: Imagem inline 2]
> Zooming...
> [image: Imagem inline 3]
>
> When I compile the map, the islands/holes are overlapped by the sea.
> I noticed that when the polygon is less complex, this overlapping
> phenomena doesn't happens. Its looks like mkgmap simplifies complex
> polygons.
>
> Is there any option that I can use in mkgmap to respect the layout of
> complex ocean polygons?
>
> By the way, if this isn't the right list to ask this kind of question,
> please let me know cause I'm new here.
>
> Thanks.
>
> Alexandre
>
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